Apparatus for detecting a magnetic field



J1me 1955 c. M. RYERSON APPARATUS FOR DETECTING A MAGNETIC FIELD 4Sheets-Sheet 1 Filed Oct. 17, 1955 FIG.1.

OSCILLATOR FIG.2.

DIFFERENTIAL PEAK AMPLIFIER OSCILLATOR June 1956 c. M. RYERSON APPARATUSFOR DETECTING A MAGNETIC FIELD 4 Sheets-Sheet 2 Filed Oct. 1'7, 1955 4Sheets-Sheet 3 m. vmm oNN moE awo H $75 12 ml yggm m l C. M. RYERSONAPPARATUS FOR DETECTING A MAGNETIC FIELD June 26, 1956 Filed Oct. 17,1955 Mk1 PFC 6 0T- June 26, 1956 c. M. RYERSON 2,752,564

APPARATUS FOR DETECTING A MAGNETIC FIELD Filed Oct. 17, 1955 4Sheets-Sheet 4 FIG.5.

APPARATUS FUR DETECTING A MAGNETIC FIELD Clitford M. Ryerson,Washington, D. C., assignor to the United States of America asrepresented by the Secretary of the Navy Original application January14, 1947, Serial No. 722,016, now Patent No. 2,727,206, dated December13, 1955. Divided and this application October 17, 1955, Serial No.544,622

7 Claims. (Cl. 32443) (Granted under Title 35, U. S. Code (1952), see.266) This invention relates to a new and improved apparatus fordetecting and measuring a magnetic field. More specifically, theinvention contemplates an arrangement in which a current is derivedproportional to the strength of the field to be measured. A secondmagnetic field is then generated from the derived current andsuperimposed upon the field to be measured, the second field beingsubstantially equal in magnitude and opposite in sign to the field to bemeasured, whereby substantially complete neutralization of the firstfield is obtained, the magnitude of the current giving an indication ofthe strength of the generated field and hence of the field which it wasdesired to measure.

This application is a division of my copending application, Serial No.722,016, filed January 14, 1947, now Patent No. 2,727,206.

One of the objects of the invention is to provide new and improvedapparatus for detecting and measuring a magnetic field.

Another object is to provide new and improved field measuring apparatusemploying the method of generating a neutralizing electromagnetic fieldequal in magnitude and opposite in sign to the field to be measured, andregistering the current required to set up the generated field, therebyto obtain an indication of the strength of the field.

Still another object is to provide a new and improved magnetometeremploying automatic neutralization of the field to be measured.

Still another object is to provide a new and improved circuit for afeedback magnetometer whereby the detector coil is also utilized forgenerating a neutralizing field.

A further object is to provide an automatic null method of measuring amagnetic field.

A further object is to provide a new and improved amplifier circuit foruse in a feedback magnetometer.

Still a further object is to provide a new and improved rectifierarrangement for deriving from an amplified A.-C. voltage a D.-C.neutralizing current proportional to the strength of the field to bemeasured.

Still a further object is to provide a circuit for obtaining arelatively large D.-C. neutralizing current whereby a neutralizing coilof a few turns may be used.

Other objects and advantages of the invention will be apparent after astudy of the specification and drawings, in which:

Fig. 1 is a schematic circuit diagram of a basic embodiment claimed inmy copending application, Serial No. 544,621, filed October 17, 1955, ofthe device;

Fig. 2 is a schematic diagram partly in block form of the circuit ofFig. l as modified to utilize the detector coils to generate theneutralizing field;

3 is schematic circuit diagram of improved amplifier and rectifiercircuit suitable for use with the bridge arranged detector coils of Fig.1;

Fig. 4 is a schematic circuit diagram of a second embodiment of theinvention in which a single pick-up or detector coil is used;

Figs. 5, 6, 7 and 8 are explanatory characteristic curves which serve toillustrate the operation of the system.

Referring now to the drawings, and more particularly to Fig. 1 thereof,the magnetic field responsive means comprises two substantially parallelcores 1t) and 11 upon which two similar coils 12 and 13 respectively arearranged to constitute a reactance device. Although the cores may bemade of any suitable magnetic material, they are preferably made ofmaterial having an extremely high permeability such, for example, as themagnetic alioy sold on the market under the trade name of Permalloy. Thecoils 12 and 13 are connected so as to have one end of each of the coilsjoined together at 14, the other ends of the coils being connected tothe respective ends of the secondary winding 15 of a transformer 16. Theprimary winding 17 is connected to an oscillator 18, the oscillatorsupplying a frequency of the order of eight hundred cycles per second tothe primary winding 17. The secondary winding 15 of the transformer hasa center tap 1d which is connected to the point 14 through primarywinding 21 of the transformer 22.

It will be noted that the elements so far described constitute a bridgecircuit with the coil 12 as one arm, the coil 13 as a second arm and theleft and right halves of the transformer secondary 15 as the third andfourth arms respectively. The transformer 22 supplies a voltagerepresentative of the state of balance of the bridge to an alternatingcurrent amplifier 25 comprising two electronic tubes ..6 and 27. Thetransformer 22 has secondary windings 23 and 24 which are joinedtogether at 28, the other end of the secondary winding 23 beingconnected to the grid 29 of the tube 26, while the other end of thesecondary winding 24 is connected to the grid 31 of the tube 27. Abattery 32 biases the two grids 29 and 31 with respect to their cathodes33 and 34 respectively. The anodes of the tubes 26 and 27 are suppliedwith a voltage from a source 35, the output from the anodes of the tubes26 and 27 being fed to the primary winding of a transformer 36. Thesecondary winding of the transformer 36 is provided with a mid-tap 37,one end of the transformer secondary winding being connected to arectifying device such, for example, as the cathode 38 of a rectifiertube 39 and the other end of the transformer secondary winding beingconnected to a similar rectifying device such as the cathode 41 of arectifier tube 42. The anode 43 of the tube 39 is connected to one plateof a condenser 44 whereas the other plate of the condenser is connectedto the mid-tap 37. The anode 45 of the tube 42 is connected to one plateof the condenser 46, the other plate of this condenser being alsoconnected to the mid-tap 37. In parallel with the condensers 44 and 46are respectively connected a pair of resistors 47 and 48. The rectifiertube 39, the condenser 44 and resistor 47 comprise a peak voltmetercircuit for the positive half cycles of the voltage supplied to thetransformer 36, the rectifier tube 42, the condenser 46 and resistor 4%performing a similar function with respect to the negative half cyclesof the voltage.

The values of the circuit components in each of these voltmeter circuitsare so adjusted that the charging rate of each of the condensersrespectively through the associated rectifier is sufficiently rapid sothat it will substantially follow the instantaneous value of thealternating voltage impressed across the rectifier and condenser whenthe voltage has the proper sign. Each condenser discharges through theassociated resistor which is of relatively large value and is soproportioned that the discharge rate will be low compared to a cycle ofthe im- 3' pressed voltage and thus the voltage across each con denserwill be substantially the peak voltage for whichever half of the cycleis passed by the associated rectifier. Since the condensers 44 and 46are connected with the voltages in opposition, a differential voltagewill appear across the points 49 and 51 which will be substantiallyproportional to the difiference in the peak voltages of the positive andnegative half cycles. This differential voltage is supplied to anamplifier 52 through a resistance coupling comprising resistances 53 and54-. One end of the resistance 53 is connected to the grid 56 of anamplifying tube 57 and one end of the resistance 54 is similarlyconnected to the grid 58 of an amplifying tube 5%. The opposite ends ofboth of the resistors are connected together and to a bias source 61.The cathodes of the tubes 57 and 59 are connected to each other and tothe bias source 61 in any conventional manner. The anodes of the tubes57 and 59 are connected to an output circuit including resistances 62,71, and 72 in parallel with meter 76 and coil 75. The variable resistor62 may be employed as a balancing means and a zero adjuster for theindicating instrument 76.

The operation of the system so far described is as follows:

The coils 12 and 13, as heretofore stated, are elements of a bridgecircuit which comprises these coils and the secondary winding 15' of thetransformer 16, the oscillator 18 serving to energize the bridgecircuit. The output terminals of the bridge circuit comprise the points14 and 19. The coils are so connected in the bridge circuit that at anyinstant the fluxes set up in the cores by the currents flowingrespectively through the coils are in opposite directions, and thus,when a substantially steady direct flux passes through the cores 10 and11 due to the earths magnetic field or any other field which it isdesired to measure, this direct flux is superimposed upon thealternating fluxes in the cores so that during one half cycle it adds tothe flux in one of the cores and ncurrently therewith opposes the fluxin the other core, while during the next succeeding half cycle the fluxto be measured opposes the flux in the said one of the cores and adds tothe flux in the said other of the cores.

The coils are so wound that the cores are operated through saturationduring each half cycle of the voltage wave. Therefore, in the presenceof a field to be measured, saturation is reached sooner during one halfcycle in each of the cores than during the next succeeding half cycle inthe same core. As a result of this the waveform of the output circuitvoltage of the bridge is rendered unsymmetrical about its zero axis;that is, one half cycle of the wave, for example, the positive halfcycle, is steeply peaked Whilst the succeeding half cycle is noticeablyflattened as shown in Fig. 6. This figure may be compared with the curvein Fig. 5 which shows the voltage wave when substantially no externalflux passes throu@ the cores 10 and 11 and the bridge is substantiallybalanced. It may here be pointed out that, whether the voltage wave issymmetrical as in Fig. 5 or unsymmetrical as in Fig. 6, the area underthe positive half cycle is substantially equal to the area under thenegative half cycle of the wave. If the steady direct flux which is tobe meaured is in the opposite direction to that which produced the curveof Fig. 6, the core which was saturated by the positive half cycle isnow saturated later than the other core so that the voltage wave isrendered unsymmetrical about its zero axis in the manner indicated inFig. 7 and the negative half cycle is sharply peaked, the positive halfcycle being flattened. It will be noted by comparing Figs. 6 and 7, thatthe peaks in these voltage waves are on opposite sides of the zero axisin the respective figures and the areas under the positive and negativehalf cycles are substantially equal.

Returning to Fig. 1, it will be seen that a voltage having either thecharacteristics of Fig. 6 or Fig. 7 appears across the points 14 and 19of the bridge 20 when a field in one direction or the other respectivelyis measured. The amplifier 25 is connected so as to amplify this voltagewave although the amplifier may be omitted or it may have as many stagesas required. The amplified voltage wave is applied to the networkincluding the rectifier tubes 39 and 42 and the rectifier tube 39 willrectify the positive half cycles of the voltage wave whereas therectifier tube 42 will rectify the negative half cycles of the voltagewave. As pointed out above, the tube 39 charges the condenser 44 and theconstants of this circuit are so adjusted that the charging rate of thecondenser will substantially follow the instantaneous peak values of thepositive half cycle voltage impressed across the rectifier. This is likewise true of the condenser 46 except that this condenser is charged tothe peak values of the negative half cycles of the voltage.

The condenser 44 discharges through its discharge resistor 47 at a ratewhich tends to maintain the voltage on the condenser 44 substantially atthe peak voltages of the respective positive half cycles. The resistor48 performs the same function with respect to the condenser 46. Thecondensers 44 and 46 are connected so that the voltage across the points49 and 51 is equal to the difference in the peak voltages of thepositive and negative half cycles of the voltage. This differentialvoltage is amplified by the amplifier 52 which may have as many stagesas desired.

Disposed adjacent the pick-up coils 12 and 13 is the additional coilwinding having its axis parallel to the axes of the coils 12 and 13 andsubstantially symmetrically spaced with respect thereto. The coil 75 isadapted to be energized by direct current from amplifier 52, thereby togenerate an electromagnetic field having its main axis in alignment withthe steady component of the field to be measured lying along thelongitudinal axes of cores 10 and 11. Coil 75 is so connected toamplifier 52 that the polarity of the field generated by coil 75 is suchas to oppose or neutralize the field to be measured. If sufiicientamplification is available in amplifier 52, the field generated by coil75 will be sutficient to substantially completely neutralize the fieldto be measured, in which case the generated field and the field which itwas desired to measure will be substantially equal in magnitude, and thestrength of the generated field will be an indication of the strength ofthe original or neutralized field.

To provide an indication of the strength of the generated field, a meter76 is provided in series with coil 75 to indicate the strength of thecurrent flowing therein. By suitable calibration, as will be obvious tothose skilled in the art, the current scale of meter 7 6 may becalibrated in suitable units of field strength such, for example, asgauss.

The direction of the current flow in coil 75 will be an indication ofthe direction or polarity of the magnetic field. Hence, if desired, themeter 76 may be of the type in which the zero reading thereof falls inthe center of the scale.

Reference is made now to Fig. 2, in which an arrangement similar to Fig.1 is shown, but in which the neutralizing current from amplifier 52fiows through the detector or pick-up coils 12 and 13. The output leadsfrom the amplifier are connected to the input leads, the output leadshaving disposed therein choke coils 77 and 78 for preventing thealternating current from the detector bridge from flowing into theoutput circuit of the amplifier. The input leads to the amplifier havedisposed therein capacitors 79 and 80 for preventing the D. C. output ofthe amplifier from getting into the input circuit thereof.

In the operation of the circuit of Fig. 2, connections are made so thatthe output current from the amplifier flows through coils 13 and 12 indirections to generate fields neutralizing or opposing the field to bemeasured. As before, the meter 76 indicating the D. C. current gives anindication of the Strength and direction of the generated field, therebyproviding an indication of the strength and direction of the field to bemeasured.

Reference is made now to Fig. 3 which shows a second embodiment of theinvention, in which the pick-up bridge is generally similar to that ofFig. 1.

In Fig. 3 however, the leads between the transformer secondary 15 andthe pick-up coils 12 and 13 have disposed therein blocking condensers 81and 84 respectively for preventing the aforementioned D. C. neutralizingcurrent applied to the detector coils from flowing through the secondarywinding, the condensers being of substantially equal capacitance. Inparallel with the pick-up coils of the bridge is a neutralizing feedcircuit comprising the two chokes or inductors 77' and 78 havingdisposed therebetween a balancing potentiometer 82, the arm of thepotentiometer being connected with the output circuit of an amplifierfor supplying neutralizing current to the detector coils, in a manner tobe subsequently more fully explained.

Two vacuum tubes 85 and 86 of the amplifier are each triple sectiontubes having a diode, a triode, and a pentode section, the diode sectionof tube 85 comprising cathode 93 and plate 98, the triode sectioncomprising cathode 93, grid 92, and anode 91, the pentode sectioncomprising cathode 93, grids 97, 96, and 95, and plate 94. Tube 86 has adiode section comprising cathode 103 and plate 108, a triode sectionincluding cathode 103, grid 102, and plate 101, and a pentode sectionincluding cathode 103, grids 107, 186, and 105, and plate 104. Whereasmulti-function tubes are shown, it is understood that separate tubescould be used if desired to supply the various sections.

The diode sections are so connected as to supply differential bias tothe control grids of the pentode sections, whereby the pentode whichamplifies one half cycle of the output voltage from the detector bridgehas a bias thereon proportional to the amplitude of the other half cycleof the output voltage.

As was previously mentioned, the pick-up or detector bridge, when in thepresence of an external field, has, due to the fact that the coressaturate sooner on one half cycle of the exciting voltage than they doon the other, an output voltage of unsymmetrical waveform in which thepeak amplitude attained by one half cycle is greater than the amplitudereached by the other half cycle. As beforementioned, Figs. 6 and 7 showthe bridge output voltage in the presence of an external field, the halfcycle having the greatest amplitude being determined by the direction ofthe steady field.

The input circuits of the diodes and pentodes are combined, to securethe beforementioned diiferential bias operation, the circuits beingtraced as follows: One of the output leads 110 from the bridge hastherein a D. C. blocking condenser 83. On the amplifier side of thiscondenser, a pair of resistances 125 and 126 are connected between fromleads 109 and 110 respectively to ground. The two cathodes 93 and 103have connected therebetween a balancing potentiometer 119 having bypasscondensers 123 and 124 connected from the arm to the ends thereof, thearm being connected to ground. The potentiometer, in addition tosupplying balancing means, may provide a steady component of bias forthe two pentode sections.

Connected in each of the output leads 109 and 110 from the detectorbridge is a storage R-C network, comprising in lead 109, the resistance117 in parallel with condenser 113, and in lead 110, the resistance 118in parallel with condenser 116, resistance 117 being connected to diodeplate 98 and resistance 118 being connected to diode plate 108. Furtherconnections of the input circuits are a connection including resistance121 between the diode plate 98 of tube 85 and the grid 107 of tube 86,and a connection including resistance 122 between diode plate 108 oftube 86 and grid 97 of tube 8 85. A coupling condenser 114 connects lead109 to grid 97 of tube 85, and a coupling condenser 115 connects lead togrid 107 of tube 86.

Assume now by way of description that the detector coils are excitedfrom source 18 and placed in an external field whereby an output voltagesimilar to that of Fig. 6 is developed across leads 109 and 110, theleads being so connected that the instantaneous positive half cycles ofFig. 6 are applied to lead 110. For the reason that the center tap ofresistances 125 and 126 is grounded, and the cathodes 93 and 94 aregrounded through potentiometer 119, the upper half cycles of Fig. 6 willtend to make plate 108 positive with respect to cathode 103 whereas thelower half cycles of the wave of Fig. 6 will tend to make diode plate 98positive with respect to cathode 93. Upon the application of a positivevoltage to plate 98, a current flows between it and cathode 93, thecurrent flowing through resistance 117 and developing a voltagethereacross which charges condenser 113.

By proper choice of component values for resistance 117 and condenser113, an arrangement may be provided in which the charging rate of thecondenser is sufiiciently rapid so that it will follow the instantaneousvalue of the peak A. C. voltage impressed thereacross, in this case, thevoltage of the lower half cycles of Fig. 6, the diode and R-C networkacting as a peak voltmeter in which the rectified voltage is developedacross the series resistance 117.

Similarly, the diode section of tube 86 conducts during the upper halfcycles of Fig. 6, developing a voltage across condenser 116 proportionalto the peak value of the half cycle. The directions of current flow aresuch that the right hand plates (Fig. 3) of condensers 113 and 116 arecharged negatively.

The voltage developed across condenser 113 by the diode of tube 85 isapplied through resistance 121 as a negative bias on grid 107 of tube86, the grid return circuit to cathode being traced as follows: grid 107through resistances 121, 117, and 125 to ground and thence throughpotentiometer 119 to the cathode of tube 86. Similarly, the voltagedeveloped by the diode of tube 86 across the condenser 116 is appliedthrough resistance 122 to grid 97 of tube 85, the grid return circuitbeing traced as follows: grid 97 through resistances 122, 118, and 126to ground and thence through potentiometer 119 to the cathode of tube85.

The control grid 97 of the pentode section of tube 85 thus has a biasapplied thereto proportional to the peak amplitude of the upper halfcycles of Fig. 6, whereas the control grid 107 of tube 86 has a biasapplied thereto proportional to the peak amplitude of the lower halfcycles of Fig. 6. However, the control grid 97 of tube 85 is coupledthrough condenser 114 to lead 109, and has the lower half cycle of Fig.6 applied thereto as a positive potential, whereas the control grid 107of tube 86 is coupled through condenser to lead wire 110, whereby theupper half cycle of Fig. 6 is applied to grid 107 as a positivepotential. It is apparent then, that differential amplification isobtained in the pentode sections, the section which has the half cycleof greatest magnitude applied thereto in a positive direction alsohaving a bias thereon proportional to the half cycle of lowestmagnitude, whereas the pentode section adapted to amplify the half cycleof smallest amplitude has a bias applied thereto proportional to thepeak voltage of the half cycle of greatest amplitude. The differentialamplification can be enhanced if desired, by the use of a tube having avariable amplification factor, for example, a variable mu pentode.

The plates 94 and 104 of tubes 85 and 86 respectively are connectedthrough blocking condensers 127 and 128 respectively to the primaries ofa pair of output transformers 133 and 134 respectively having the otherends of the primary windings grounded at 129. Between the plates 94 and104 are connected in series resistances 131 7 and 132, the common pointbetween resistances being connected to a suitable source of platepotential. The outputs of the pentode sections are, accordingly,delivered to the respective output transformers.

It is observed that the plate 104 of tube 86 is connected to the screengrid 96 of tube 85, and that the plate 94 of tube 85 is connected to thescreen grid 106 of tube 86. As will be understood by those skilled inthe art, these connections provide an arrangement in which the gain oftube 85 is modified by the intensity of the signal amplified by tube 86,an increase in the amplitude of the signal applied to tube 86 resultingin an increase in plate current in that tube, and a fall in the platevoltage thereof and in the screen voltage of the other tube, therebycausing a reduction in the gain of tube 85 and a corresponding reductionin output. Similarly, the gain of tube 86 is modified by the intensityof the signal amplified by tube 85, an increase in the amplitude of thesignal applied to tube 85 resulting in a reduction in the gain andoutput of tube 86. There is provided then, additional differentialaction supplementing the aforedescribed differential bias.

The double diode tube 135 has the plates 137 and 138 thereof connectedto one end of the secondary windings of output transformers 133 and 134respectively, the oth' er ends of the secondaries being connectedthrough resistances 141 and 142 respectively to cathode 136, resistances141 and 142 being paralleled by condensers 143 and 144 respectively.Thus, this circuit provides an arrangement whereby, if the circuitconstants are chosen properly, a rectified voltage is developed acrosscondenser 143 proportional to the peak voltage output of transformer133, and a rectified voltage is developed across condenser 144proportional to the peak voltage output of transformer 134.

The triode sections of tubes 85 and 86 are connected as D.-C.amplifiers, the rectified output from diode plate 137 and transformer133 being applied to grid 92, the rectified output from diode plate 138and transformer 134 being applied to grid 102. The grid cathode circuitof grid 92 includes resistance 111 in parallel with condenser 87, andthe grid cathode circuit of grid 102 includes resistance 112 in parallelwith condenser 88. It is noted that there is a complete electricalcircuit for the passage of direct current, this circuit being traced asfollows: from resistor 141 through resistance 111, through potentiometer119, resistance 112, and thence through resistance 142 to the other endof resistance 141. The diode sections of tube 135, in conjunction withthe triode sections of tubes 85 and 86, constitute a differential peakamplifier generally similar to that disclosed in Fig. l and embracingtubes 39, 42, 57, and 59 therein. The plates 91 and 101 of therespective triode sections are connected as shown to two resistances 145and 146 having the other ends thereof connected together and to asuitable source of plate energizing potential with the condenser 147connected between plates. The plates 91 and 101 are also connected byleads 152 and 151 respectively to the arm of potentiometer 82 and to thecenter point 14 of the detector coils respectively. Lead 152 has themeter 76 therein for indicating the value of the neutralizing current.

The operation of the circuit of Fig. 3 will be readily understood inview of descriptions aforegiven of the operation of the circuits ofFigs. 1 and 2. The application of a signal to the aforedescribed inputcircuit causes a D.-C. current to flow through the leads 151 and 152,and pick-up coils 12 and 13, the direction of flow depending upon whichchannel including the pentode sections has the signal or half cycle ofgreatest peak amplitude ap plied thereto. By suitable choice of circuitconnections, the fields generated in the pick-up coils by the D.-C.current therein may be made of proper polarities to oppose the field tobe measured. Provided suflicient gain is available in the amplifiers,substantially complete neutralization of the field to be measured may beobtained, the direction and amount of D.-C. current flow as registeredon meter 76 giving an indication of the direction and magnitude of thegenerated field and hence of the field which it was desired to measure.

Reference is made now to Fig. 4, which shows a feedback magnetometeremploying a single pick-up or detector coil, and a modified amplifiercircuit somewhat similar to that of Fig. 3, and suitable for developinga neutralizing current from the signal generated in the single coil. Theoscillator 18 delivers its output to a transformer 161 having primarywinding 162 and secondary winding 163. The output of the transformer,which should be of substantially pure waveform for reasons heretoforeexplained, is delivered through series resistance 164 to the pick-upcoil 180 having a saturable core 181.

As previously explained, when a saturable core is driven to saturationperiodically by an alternating current of substantially sinusoidalwaveform, harmonics are introduced in the Waveform of the excitingcurrent. If there is a steady component of a magnetic field along theaxis of the core, this component will oppose the saturating field duringone half cycle and add to the saturating field during the other halfcycle. As a result, the core will saturate sooner on one half cycle thanon the other, the harmonic distortions of the wave will be different inalternate half cycles, and an asymmetry will be introduced in thewaveform in which one half cycle has a peak amplitude greater than theother half cycle. Such a waveform is illustrated in Fig. 8, in which theunbroken trace represents the distortion of the exciting currentproduced in coil 180 due to the presence in the core of a steadycomponent of a magnetic field. Whether the upper or lower half cycleexceeds the other in amplitude depends upon the direction of themagnetic field along the core. The presence of the resistance 164 in thecircuit insures that the impedance of the generator is maintainedrelatively high with respect to the impedance of the load (coil 180), sothat distortions of the Waveform occurring in the coil appear insubstantial magnitude across the input circuit of the vacuum tubeamplifier.

Each of the two vacuum tubes 185 and 186 is a triple section tube,comprising a diode, a triode, and a pentode section. Tube 185 has diodeelements including cathode 193 and diode plate 198, a triode sectionincluding cathode 193, grid 192, and plate 191, and a pentode sectionincluding cathode 193, control grid 197, screen grid 196, suppressorgrid 195, and plate 194. Tube 186 has a diode section including cathode203 and diode plate 208, a triode section including cathode 203, controlgrid 202, and plate 201, and a pentode section including cathode 203,control grid 207, screen grid 206, suppressor grid 205, and plate 204.

The output leads 165 and 166 from the detector coil have connectedthereacross resistances 167 and 168 in series, the common terminal pointbetween resistances being connected to ground. The cathodes 193 and 203of the tubes and 186 are connected through potentiometer 173 having thearm thereof connected to ground and having condensers 171 and 172connected between the arm of the potentiometer and the ends thereofrespectively. The potentiometer provides means for balancing the gain oftwo signal channels hereafter to be described, and also provides asteady component of bias for the tubes, as will be hereafter apparent.

The output leads 165 and 166 are connected through condensers 215 and216 respectively to the control grids 197 and 207 respectively of thepentode sections, grid return to ground from grid 197 being throughresistances 221 and 217, and grid return to ground from grid 207 beingthrough resistances 222 and 218. By reason of the voltage divider orcenter tap connection between resistances 167 and 168, the pentodesections 'of tubes 185 and 186 each acts as a signal channel for onehalf cycle of the output voltage of coil 180.

The plates 194 and 204 have series resistances 223 and 224 connectedtherebetween, the common point between resistances being connected to asuitable source of plate potential, the resistances providing loadsindividual to the pentode tube plates across which the outputs aredeveloped. Condensers 187 and 188 couple plates 194 and 204 respectivelyto the grids 192 and 202 respectively of the triode sections, gridreturns to cathode being provided by resistances 211 and 212respectively. The signal outputs of the pentodes are amplified in thetriode sections in a conventional manner. The plates 191 and 201 of thetriodes are connected through series connected resistors 219 and 220,the common point between resistances being connected to a suitablesource of plate potential, outputs of the triode sections beingdeveloped across these respective load resistances. Plate 191 isconnected to screen grid 196 for reasons to be subsequently explained,and is coupled through condenser 214 to diode plate 208. Similarly,plate 201 is connected to screen 206 and is also coupled throughcondenser 213 to diode plate 198.

The circuit operates to provide differential bias for the pentodesections, thereby to provide unequal amplification for the two alternatehalf cycles of the input signal, depending upon which half cycle is ofgreater amplitude.

Referring again to Fig. 8, in which the unbroken line represents thesignal from the detector coil 180, by reason of the center taparrangement of resistances 167 and 168, at the moment a voltage of givenpolarity and magnitude resulting from one half cycle of the signal isapplied between one pentode control grid and cathode, a voltage of equalmagnitude and opposite polarity is applied to the control grid of theother pentode section, these voltages being indicated by the dashedlines of Fig. 8. As aforedescribed, the circuit provides an arrangementin which the signal applied to control grid 207 is amplified in thepentode and triode sections of tube 186 and applied through condenser213 to the diode plate 198 of tube 185, where rectification occurs, theresulting D.-C. potential being applied through resistance 221 to grid197. Assume now by way of description that the first half cycle of thesignal voltage of Fig. 8 is applied to the voltage divider networkincluding resistances 167 and 168 whereby grid 197 is made positive withrespect to cathode at the same moment the image voltage represented bythe other dashed line makes grid 207 negative with respect to cathode.The current through the pentode plate 204 decreases, raising the platepotential, causing the potential on the coupled triode grid 202 toincrease, increasing the triode plate current, causing the potential onplate 201 to fall. This decrease in voltage is reflected throughcondenser 213 to anode plate 198, tending to diminish the rectificationoccurring therein. As aforementioned, by reason of the resistance 221linking diode plate 193 and grid 197, the rectified voltage developedacross resistance 217 by the diode is applied as a negative bias on grid197. It should be noted also, that the signal voltage applied to grid197 through condenser 215 is also applied to the diode plate 198 throughresistance 221, tending to develop a rectified voltage across resistance217, but of such limited value in comparison with the amplified voltageapplied to the diode through condenser 213 that the effect of the signalvoltage in developing a bias may be neglected. By proper choice ofcomponent values, it will readily be understood that conditions areprovided in which the instantaneous bias on the grid 197 of the pentodeis reduced proportional to the amplitude of the half cycle exciting thegrid 197 of the pentode in a positive direction, resulting in aninstantaneous increase in gain.

The next succeeding half cycle of the signal voltage of Fig. 8 isapplied as a potential on the pentode grid 207 positive with respect tocathode or ground. Simultaneously therewith, it causes a negativepotential with respect to ground to be applied on grid 197. The circuitsof tube operate to reduce the bias on grid 207 of tube 186 proportionalto the instantaneous value of the peak voltage of the half cycle. It isapparent then, that differential amplification in the two signalchannels is obtained, that channel amplifying the half cycle of greateramplitude having the greater gain. By reason of the intervening stagesof amplification provided by the triode sections, the differential biaseffect is magnified and enhanced.

The pentode plates 194 and 204 are also observed, Fig. 4, to be coupledthrough condenser 225 and 226 respectively to the control grids 229 and230 respectively of a double triode tube 227, grid returns beingprovided by resistances 233 and 234 respectively, the tube having plates231 and 232 respectively and cathode 228. After amplification in thedouble triode, the outputs of the plates 231 and 232 are applied to theprimary windings of output transformers 235 and 236 respectively.

Each of the transformer outputs is fed to a full wave rectifier, therectifier associated with transformer 235 comprising rectifier elements237, 238, 239, and 240, the elements being of the copper oxide type orany other suitable type, the rectifier associated with transformer 236comprising elements 241, 242, 243, and 244. The output terminals of thefull wave rectifiers are connected to the neutralizing solenoid 75having current indicating means 76 in circuit therewith. It will beapparent to those skilled in the art to which the invention pertainsthat, upon energi-zation of the amplifier channels from coil 180, arectified D.-C. current will flow in coil 75, the direction of currentflow-depending upon which full wave rectifier has the greater output,and hence upon which signal amplifier channel has the greateramplification. As before stated, this latter is determined by thatsignal channel having the half cycle of greater peak magnitude appliedthereto. By proper circuit arrangement, the current flowing in coil 75is made to set up an electromagnetic field opposing the field to bemeasured. When the generated field is of sufiicient amplitude tosubstantially null or cancel the field which it is desired to measure,the current indicated by meter 76 will indicate the strength of thefield which it is desired to measure.

Any suitable means may be provided for heating the filaments or heatersof the various tubes. Whereas triodes, pentodes, or diodes are shown, itis understood that tubes employing other arrangements of elements may beemployed, the functions of the various sections remaining the same. Whencommon plate supplies are used for the various tubes, decoupling meansmay be employed if desired, or separate supplies used.

It is understood that additional amplifiers may be added if desired.

Whereas the invention has been shown and described with reference tospecific embodiments thereof which give satisfactory results, it isobvious that various modifications of form or structure may be madewithout departing from the spirit or scope of the invention, and Itherefore include all such modifications and equivalents in the appendedclaims.

The invention described and claimed herein may be manufactured and usedby or for the Government of the United States of America without thepayment of any royalties thereon or therefor.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. In a feedback magnetometer employing means for introducingasymmetries in alternate half cycles of an alternating current Waveproportional to the magnitude of the field to be measured andselectively in accordance with the sign of said field, and meansresponsive to said asymmetries for generating a neutralizing fieldsubstantially equal in magnitude but opposite in sign to said firstnamed field, means for amplifying the components of voltage resultingfrom said asymmetries and deriving a direct neutralizing currenttherefrom, said means comprising a pair of thermionic rectifiers forproducing rectified voltages proportional respectively to the peakvalues of alternate half cycles of said voltage, a pair of thermionicamplifiers adapted respectively to amplify alternate half cycles of saidvoltage, means for differentially biasing said thermionic amplifiersfrom the rectified voltages developed by said rectifiers whereby theamplifier energized by one half cycle has its bias controlled by thepeak value of the other half cycle, a pair of rectifier meansindividually energized by said amplifier tubes, and a circuit forcombining the outputs of said rectifier means to obtain saidneutralizing current.

2. A feedback magnetometer comprising, in combination, an electricbridge circuit including a pair of detector coils, means for supplying aperiodically varying voltage having a substantially symmetrical waveformto said circuit, means responsive to a magnetic field and connected insaid circuit for changing the symmetry of the Waveform of said voltageto produce peak voltages on one side of the zero axis thereof which areof greater magnitude than the peak voltages on the other side of thezero axis in accordance with the strength and direction of the externalfield, a pair of thermionic rectifiers connected to said circuit andadapted to produce rectified voltages proportional respectively to thepeak voltages on opposite sides of the zero axis, a pair of thermionicamplifier tubes each connected to said circuit and adapted to amplifyselectively said peak voltages, means for dilferentially biasing saidthermionic amplifiers from the rectified voltages developed by saidrectifiers whereby the amplifier energized by the voltage on one side ofthe zero axis has its bias controlled by the voltage on the other sideof the zero axis, a pair of rectifier means energized by said amplifiertubes, a circuit for combining the outputs of said rectifier means, aneutralizing coil disposed in predetermined relation to said detectorcoils and adapted to be energized by said combined outputs to generatean electromagnetic field substantially equal in magnitude and oppositein sign to the field to be measured, and means controlled by thecombined outputs for indicating the magnitude of the generatedelectromagnetic field.

3. In apparatus of the character described for measuring the magnitudeof a magnetic field, a source of voltage of substantially sinusoidalwaveform, a detector energized from said source and adapted to have acomponent of voltage generated therein proportional to the strength ofthe field to be measured, said component resulting in distortions ofsaid waveform whereby alternate half cycles are rendered of unequal peakamplitude selectively in accordance with the polarity of the field to bemeasured, amplifier means excited by said voltage of distorted waveform,said amplifier means including a pair of rectifiers each adapted to passone half cycle of the exciting voltage, an R-C network associated witheach rectifier for securing voltages proportional to the peak amplitudesof the half cycles of voltage passed by the respective rectifiers,push-pull vacuum tube means wherein the control grids of the tubesthereof have both the last named voltages simultaneously applied thereonin opposite senses respectively, connections between said vacuum tubemeans and said detector for causing a direct current flow in saiddetector which sets up a field opposing the field to be measured andsubstantially equal in magnitude, and means controlled by said currentfor indicating the magnitude of the generated field.

4. In apparatus of the character disclosed for measuring a magneticfield, detector means adapted to be disposed within the field, a sourceof voltage of substantially sinusoidal waveform connected to saiddetector means for exciting the same, said detector means being adaptedto introduce distortions in the waveform of the exciting voltageproportional to the strength of the component of the magnetic fieldlying along the major axis of the detector, said distortions resultingin differences in the peak amplitudes of the alternate half cycles ofthe exciting current selectively in accordance with the polarity of thefield, an output circuit connected to said detector means, a pair ofthermionic rectifiers connected in said circuit, each of said rectifiersbeing adapted to rectify one half cycle of the detector output voltagein said circuit, an R-C network associated with each of said rectifiersand adapted to develop rectified voltages thereacross proportional tothe peak amplitudes of alternate half cycles of the voltagerespectively, a pair of thermionic amplifier tubes in pushpullarrangement and each adapted to amplify one half cycle of said detectoroutput voltage, connections whereby the rectified voltage developedduring one half cycle of the detector output voltage is applied as biasto the amplifier tube which amplifies the other half cycle respectively,a pair of rectifier means energized by said amplifier tubes, a circuitcombining the outputs of said rectifier means, a neutralizing coildisposed in predetermined relation to said detector and adapted to beenergized by said combined outputs to generate an electromagnetic fieldsubstantially equal in magnitude and opposite in sign to the field to bemeasured, and means controlled by the combined outputs for indicatingthe magnitude of the external field.

5. A magnetic field direction responsive system of the characterdisclosed, comprising, in combination, a field detector, means forsupplying a periodically varying voltage having a substantiallysinusoidal waveform to said detector, means associated with saiddetector for changing the symmetry of the waveform of said voltage toproduce peak voltages on one side of the zero axis thereof which are ofgreater magnitude than the peak voltages on the other side of the zeroaxis in accordance with the direction and strength of the externalfield, dual amplifier means connected to said detector for separatelyamplifying the alternate half cycles of said voltage, dual rectifiermeans individually energized from said dual amplifier means for derivingtwo D.-C. potentials proportional to the peak amplitudes of therespective half cycles of said voltage, connections between said dualrectifier means and said dual amplifier means whereby the amplifiermeans passing one half cycle has its conductance altered in an amountproportional to the peak voltage of the other half cycle, a commonoutput circuit for said dual amplifier means including second rectifiermeans, said second rectifier means being adapted to generate a currentproportional to the strength of the field, a neutralizing coil disposedin predetermined relation to said detector and adapted to be energizedby said current to produce an electromagnetic field equal in magnitudebut opposite in sign to said first named magnetic field, and meanscontrolled by said current for indicating the strength of the externalfield.

6. In a feedback magnetometer of the character disclosed, means forgenerating a signal representative of the magnitude and polarity of amagnetic field, said signal being characterized by an asymmetry in whichthe positive and negative half cycles with respect to the zero axis areof unequal amplitude selectively in accordance with the polarity of thefield and in amounts proportional to the magnitude of the field, meansfor deriving two D.-C. potentials each proportional to the peakamplitude of one half cycle of said signal, dual amplifier means eachadapted to amplify one half cycle of said signal, means for applyingsaid D.-C. potentials to said dual amplifier means whereby the amplifierresponsive to the half cycle of greater amplitude has a negative biasvoltage applied thereto proportional to the half cycle of smalleramplitude and the amplifier responsive to the half cycle of smalleramplitude has a negative bias voltage applied thereto proportional tothe half cycle of greater amplitude, and means energized from said dualamplifier means for generating a field of substantially equal magnitudeand opposite polarity to the field to be measured.

7. In a feedback magnetometer of the character disclosed, means forgenerating a signal representative of the magnitude and polarity of amagnetic field, said signal being characterized by an asymmetry in whichthe positive 13 and negative half cycles with respect to the zero axisare of unequal amplitude selectively in accordance with the polarity ofthe field and in amounts proportional to the magnitude of the field,dual amplifying means for separately amplifying alternate half cycles ofthe signal, means for individually rectifying the outputs of said dualamplifying means, circuit means for applying the rectified outputs asbiases to the dual amplifying means in a manner such that the amplifierresponsive to the half cycle of greater amplitude has a negative biasvoltage applied thereto proportional to the half cycle of smalleramplitude and the amplifier responsive to the half cycle of smalleramplitude 14 has a negative bias voltage applied thereto proportional tothe half cycle of greater amplitude, and means energized from said dualamplifier means for generating a field of substantially equal magnitudeand opposite polarity t0 the field to be measured.

References Cited in the file of this patent UNITED STATES PATENTS

